In mining operations, such as coal mining, the roof may be supported by roof bolts that are embedded within spaced apart holes that have been drilled in the mine roof. A common roof drill bit design uses a cutting insert that has been brazed or otherwise attached into a slot at the axially forward end of the roof drill bit body.
The roof drill bit is then pressed against the roof and drilling apparatus operated to drill a bore hole in the roof. The bore hole may extend between two feet to greater than twenty feet into the roof. The bore hole is then filled with resin or other grout material and the roof bolt is affixed with the bore hole. A roof support, such as a plate or roof panel, may be attached to the roof bolt.
A fully grouted bolt anchors itself by frictional interlock between the resin and the rock. Thus, drill bit manufacturers may use wide tolerances or offset bit cutters to induce wobble during drilling which, when combined with loose bit mounting to the drill rod produces ridges on the hole walls. The ridges produce a wall roughness that may increase anchoring capability.
Another method of producing wall roughness is to use a drill bit with cutting members arranged in a helical pattern on the periphery of a central hub to form a helical grove or thread. This drill bit, however, must follow a standard cutting bit, which requires an additional process step. This additional process step is generally undesirable since it increases the roof bolt installation time. One solution that has been proposed is to use a helical roof bolt that will cut a helical groove in the borehole as part of the roof bolt installation process (which occurs after the bore hole is created). A problem with this approach is that the cuttings produced during installation of the roof bolt may contaminate the resin and reduce the cross-sectional area available for the resin to bond.
The present invention provides a cutting drill bit that can create a helical or rifled pattern on the bore hole wall in a single pass. Advantageously, practice of the invention may include the use of resin or it may be dispensed with. In addition, the principles of the present invention are advantageously used with both vacuum drill bits and wet drill bits.